Purpose :
Visual counting requires rapid shifts in attention in the visual field (Anobile et al., 2012; Egeth et al., 2008), and this process is drastically limited with the amblyopic eye viewing in strabismic amblyopia (Sharma et al., 2000). We speculated that the performance of visual counting might be further affected under binocular viewing when shifting attention between the eyes in amblyopia.

Methods :
We measured visual counting in each eye of adult amblyopes and normal-vision observers, using a variant of the Sharma paradigm (Sharma et al., 2000) modified for a binocular approach. Through a mirror stereoscope, highly visible Gabors (2cpd, high contrast, 200 ms duration) were presented within the central visual field, preceded with 500 ms cue (5.6 ° square frame) that cued which eye received the Gabors, followed by 200 ms noise mask. The contrast of the Gabors was balanced between the two eyes. In the first experiment, the Gabors were presented to only one eye. In the second experiment, the Gabors were randomly assigned to either the right or the left eye each trial. The subjects were instructed to report the number of Gabors (0 to 9) as quickly and accurately as possible. No feedback was given. Visual counting performance was compared between the amblyopic eye and the fellow eye of amblyopes, and also between the amblyopes and normal-vision observers.

Results :
In the first experiment, adult amblyopes undercounted the number of Gabors when viewing with the amblyopic eye, but achieved near-perfect performance when viewing with the fellow eye as compared to normal-vision observers. This result is consistent with the reports from previous monocular studies. In the second experiment, when shifting attention randomly to the left or to the right eye by the cue, amblyopes dramatically undercounted the number of Gabors in both the amblyopic and the fellow eyes. Normal-vision observers experienced no difference when shifting attention between the eyes, as compared to monocular viewing.

Conclusions :
In addition to the observation that amblyopes underestimated visual counting with their amblyopic eye, our data showed that visual counting performance was further affected under binocular viewing when shifting attention between the eyes in amblyopes. Our results provided evidence further supporting the view that the ability to quickly redirect attention is impaired in amblyopia (Lai et al., 2013).

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.